It can also be used for focus stacking or astrophotography. Stacking, as a technique/term, is broader than just exposure fusing/HDR.
See the Hugin "Creating 360º enfused panoramas" tutorial. Alternatively, you can provide control points and align images. You can do this in the Photos tab by right-clicking on the image, and selecting Stacks → Change stack… and manually setting the stack number for the image. If you did, however Hugin's saying you'll have to define which images belong together as a stack manually, rather than relying on automated identification of the stack. If you weren't feeding it exposure-bracketed sets, then don't worry. What that message basically means is that you've added a set of images with a wide enough dynamic range that it's looking to see if you fed it bracketed sets of images but it can't detect enough similarity between specific images to organize them into exposure stacks. Hugin will let you feed in all 12 shots at once, and organize them into four stacks of three exposures each. But you want to do HDR, so you bracket shots at ☒EV, so you have 12 shots (3 bracketed shots of each of the four images). So, say, you'd need four images to cover your pano scene. Stitching 360x180 panoramas shot outside almost inevitably means you have the sun in your shot and are covering a very large dynamic range, so pano shooters were among the first users to do HDR processing. Otherwise, align_image_stack will process the brightest image first, and the darkest one last - something you want to do in HDR photography, but something you don't want when you have, e.g., 20 images of equally exposed images of the night sky.Many people use Hugin to do not just panostitching, but also exposure stacking (for high-dyanmic range (HDR) or exposure fusion) at the same time. Remember to set -use-given-order when your pictures are +/- equally bright. They give the program large room for "correct" alignments, but with sometimes drastically distorted images. Use other options (like -m -d -i -x -y -z, for optimizing several things) only when you need them - and they're rarely needed when you photograph distant objects like stars or landscapes. Remember to increase the number of control points because the cells get larger (see 1.). g 3 or -g 2 for a 3x3 or 2x2 grid, respectively, can be useful for difficult images. The standard value is -g 5, so the software looks for common points in a 5x5 grid. If there is a rather large shift between your pictures, you can try lowering the grid size. The default are three pixels ( -t 3), but usually applying steps 1. (Conversely, you can increase the correlation value to 0.95 or 0.99 if alignment is too easy to begin with.) Of course, that way the program will also include very bad matches - but this is usually offset by the much larger number of control points. The standard value is -corr=0.9, so if align_image_stack still fails to align images in spite of the higher number of control points, you can lower this to -corr=0.8 or even down to -corr=0.5. Play around with the required correlation between control points. You will not get a worse result because of that - align_image_stack will just use (much) more time. Raise this number to 20, 50, 100 or even 500.
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